This invention relates to the fabrication of bioprosthetic heart valve replacements. Valve replacements are required for patients having a heart valve which is diseased or otherwise incompetent. Commonly, heart valve prostheses are made from a combination of either human or animal tissue and mechanical elements. These so-called "bioprostheses" have several advantages over purely mechanical valves or valves made only from tissue. Like mechanical valves, they are more durable than tissue valves but unlike mechanical valves they do not generally require that the patients undergo anticoagulant therapy for the rest of their lives.
U.S. Pat. No. 5,163,955 (the '955 patent), assigned to Autogenics, assignee of the present application, discloses such a bioprosthetic valve invention in which an inner stent, on which the autologous tissue used to construct the valve is wrapped, is inserted into a spreadable outer stent containing a self-adjusting tensioning spring around the circumference of its base. The spread outer stent clamps the stents together at its base and at a plurality of posts projecting from the bases of both the inner and outer stents. This clamping thus secures the tissue while compensating for irregularities in the tissue and supplying a clamping force which is evenly distributed over the entire circumference of the tissue.
The '955 patent, which is incorporated herein by reference, also discloses assembly tooling for mating the inner and outer stents. For the assembly method described in the '955 patent, a sleeve, a top spreading tool, and a spreading bullet are used to spread the outer stent and insert the tissue-wrapped inner stent into engagement with the inner surfaces of the outer stent. In use, the outer stent is pushed, bottom first, over the top of the spreading bullet. The bullet's ramped surfaces cause the outer stent to spread open until the bottom of the annular base of the outer stent rests on the ledge of the sleeve, which fits over the spreading bullet. The sleeve, which now holds the spread outer stent, is then removed from the spreading bullet. A mandrel holding the inner stent is inserted into the center of the sleeve, completing the assembly.